Monitoring blackleg races to inform R gene development

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KEY RESULT:

The avirulence profile of L. maculans population on the Prairies remained relatively stable from 2017 to 2021; only AvrLep1 increased noticeably. Even so, the current pathogen population is still highly diverse. Researchers should continue monitoring the pathogen population for early detection of resistance erosion to specific R genes, especially when new R genes are being used in canola cultivars.

PROJECT TITLE, PRINCIPAL INVESTIGATOR:

“Monitoring the race dynamics of Leptosphaeria maculans for effective deployment and rotation of resistance genes for sustainable management of blackleg of canola in western Canada,” Gary Peng, Agriculture and Agri-Food Canada, Saskatoon

FUNDING:

Alberta Canola, SaskCanola, Manitoba Canola Growers

PUBLISHED ARTICLES:

Soomro W, Kutcher HR, Yu F, Hwang SF, Strelkov SE, Fernando WGD, McLaren D, Peng G. 2021. Race structure of Leptosphaeria maculans in western Canada between 2012 and 2014 and its influence on blackleg of canola. Can J Plant Path. 43: 480-493.

Liu F, Zou Z, Peng G, Fernando WGD. 2021. Leptosphaeria maculans isolates reveal their allele frequency in western Canada. Plant Disease 105: 1440–1447.

For canola cultivars with qualitative, or major-gene, blackleg resistance, the plant may have a gene that recognizes one of the common Leptosphaeria maculans races present in a field. When that happens, the race is considered to be carrying an avirulence gene, which triggers a resistant response in the plant that prevents the pathogen from causing blackleg. If the plant doesn’t carry an R gene corresponding to any of the common pathogen races present in a field, these races are considered virulent, and the plant is likely somewhat susceptible to infection. (All current canola cultivars carry a level of quantitative resistance to blackleg.)

Analysis of avirulence-gene frequency in the L. maculans population provides important guidance to the selection of effective R genes in blackleg resistance breeding, as well as to the recommendation of canola cultivars carrying effective R genes.

Figure 1. Avirulence (Avr) gene frequencies in the population of Leptosphaeria maculans on the Canadian Prairies in 2021. The higher an Avr-gene frequency, the greater chances for the corresponding R genes to be effective in a region. For example, the figure shows that the avirulence gene AvrLm1 was moderately high (>50%) in Manitoba, while extremely low in Alberta and Saskatchewan. This means that cultivars carrying the R gene Rlm1 or LepR3 alone can be fairly effective in Manitoba, but not so much in the other two provinces.

This study identified the pattern of avirulence dynamics in the
L. maculans population in Western Canada between 2017 and 2021. This prompted the deployment of new R genes in canola cultivars.

Key objectives were to (1) monitor and analyze the pathogen population to gain insights into race dynamics and population virulence; (2) provide industry and producers with up-to-date pictures on L. maculans population virulence in the prairie provinces to guide the selection and recommendation of effective R genes for blackleg resistance breeding and disease management; (3) understand the diversity of L. maculans population and identify pathogen races capable of overcoming common R genes in canola cultivars before widespread breakdown of the resistance; and (4) evaluate, develop and adopt markers for efficient L. maculans race monitoring.

Each year, provincial disease survey teams collected canola stubble with blackleg symptoms, and researchers tested these samples for the presence or absence of known avirulence (Avr) genes.

The study showed that canola cultivars carrying any of the resistance genes Rlm5, Rlm6, Rlm7, Rlm10, Rlm11 and LepR1 are likely resistant to blackleg in Western Canada due to common presence of corresponding Avr genes in the pathogen population. The resistance genes Rlm3 or Rlm9 may be ineffective due to the masking effect of AvrLm7 on the AvrLm3 or AvrLm9, and Rlm4 may be less effective in Alberta due to low levels of the avirulence gene AvrLm4.

A total of 83 races were identified in the L. maculans population, with top-10 races accounting for 72.3 per cent of the population in Alberta, 93.7 per cent in Saskatchewan and 57.1 per cent in Manitoba. The diversity in pathogen shows the potential for resistance erosion as virulent races had already been present for all known R genes, except Rlm10. To manage selection pressure that can lead to resistance breakdown, judicious uses of R genes will be important.